The early years of the Space Age were filled with overly ambitious plans with schedules that proved to be far too optimistic given the state of the technology of the time. It was only after the true difficulty of a development effort became apparent that projects would have to be descoped or cancelled outright. A perfect example of this was NASA’s early Mariner program to explore Venus and Mars which, at the beginning, envisioned far more ambitious goals than the flights actually flown in the 1960s and early 1970s.
The Start of the Mariner Program
In December 1959, NASA Headquarters directed Caltech’s Jet Propulsion Laboratory (JPL) to develop plans to explore the Moon and planets. Since JPL had already played a role in the development of some of America’s first Earth satellites as well as built the Pioneer 4 probe which made a distant flyby of the Moon in March 1959, the organization was uniquely qualified for the task (see “Explorer 1: America’s First Satellite“, “Vintage Micro: The Second-Generation Explorer Satellites” and “Vintage Micro: The Pioneer 4 Lunar Probe“). This effort led to the start of the Ranger program to impact probes on the lunar surface (see the Ranger Program Page) as well as the more advanced Surveyor program to study the Moon with soft landers as well as from orbit (see “Surveyor 1: America’s First Lunar Landing”). Planetary exploration, which would initially target Venus and Mars, would be the goal of the Mariner program. Mariner A would be tasked with performing an initial flyby reconnaissance of Earth’s nearest planetary neighbors followed by Mariner B missions which would drop entry probes or even simple landers to provide in situ measurements of Venus and Mars as the main bus flew by and studied the target planet remotely. These missions would then be followed by the larger and more capable Voyager (not to be confused with the later series of the same name which explored the outer planets). Voyager would drop soft landers on the surface of Venus and Mars (see “The Automated Biological Laboratory“) as well as potentially study the planets from orbit.
A diagram of an early Mariner A concept from July 1960. Click on image to enlarge. (NASA/JPL)
The launch vehicle for these half-metric ton Mariner spacecraft was to be the Atlas-Centaur whose development had been officially transferred to NASA on July 1, 1959. Unlike earlier rockets, the Centaur was fueled by cryogenic liquid hydrogen which was half again as efficient as other fuels in use at the time like alcohol or kerosene. Since the Centaur was also intended to be used as the third stage of the Saturn heavy lift launch vehicle (see “The Birth of NASA’s Saturn C-1 Rocket”) being developed at what would become NASA’s Marshall Space Flight Center (MSFC), NASA headquarters gave MSFC responsibility for the Centaur development in July 1960 just as the exact goals and schedule for the first Mariner missions were being debated. The 3-meter in diameter Atlas-Centaur consisted of two stages connected by an interstage adapter. The first stage was a modified Atlas D ICBM which used special grade of kerosene known as RP-1 and LOX as propellants for its three main engines and pair of vernier engines. With a total length of 20 meters including the interstage adapter, the modifications to the Atlas included a redesigned forward section to accommodate the Centaur which was substantially wider and heavier than the upper stages used earlier with the Atlas.
Early concept drawing for the Atlas-Centaur rocket which would serve as the launch vehicle for the proposed Mariner A. Click on image to enlarge. (NASA)
The new Centaur second stage had a total length of 14 meters including its nose shroud. It used a pair of Pratt & Whitney RL-10 engines burning liquid hydrogen and LOX to generate 67 kilonewtons of thrust each. Because the types of orbits for NASA’s proposed missions required a restart capability, Centaur included an attitude control system and ullage jets powered by hydrogen peroxide to keep the stage oriented properly and its propellants settled in their tanks during the weightless coast between engine burns. In order to minimize the boil off the cryogenic liquid hydrogen fuel, Centaur was fitted with insulation panels which were jettisoned after the rocket had left Earth’s atmosphere and were no longer needed in order to help improve the launch vehicle’s payload capability.
The Mariner A Spacecraft
By the summer of 1961, JPL’s plans called for the launch of a Mariner A spacecraft towards Venus on the second Atlas-Centaur flight during a window up to 67 days long centered on August 1962. The close encounter with Venus would take place in mid-December after a transit time of 91 to 158 days (with shorter transit times coming later in the launch window). This would be followed by a Mariner B mission to drop a probe on Mars scheduled for launch in November 1964.
At the heart of the 500-kilogram Mariner A was a two-tier, hexagonal frame which would house the spacecraft’s vital systems. Unlike the earlier spin-stabilized Pioneer probes (see “Vintage Micro: The First Interplanetary Probe”), the Mariners were three axis stabilized to provide a stable platform with a fixed orientation for its suite of instruments. The attitude would be controlled by a set of cold gas jets with celestial attitude references provided by a Sun sensor as well as an Earth sensor mounted on the spacecraft’s hinged high gain antenna attached at the bottom of the frame. Mounted on opposite sides of the spacecraft were a pair of deployable solar panels with an area of 1.85 square meters each. These panels would be kept pointed at the Sun to provide 300 watts of electrical power at the Earth and 600 watts when near Venus to recharge the spacecraft’s batteries. At the heart of the frame was a hydrazine monopropellant engine to provide up to two course corrections during the flight to Venus to fine tune its aim.
Diagram showing the major components of the Mariner A concept as it was envisioned by the summer of 1961. Click on image to enlarge. (NASA/JPL)
The proposed instrument suite for Mariner A included a set of radiometers to determine the temperature of Venus and an ultraviolet spectrometer to study sunlight reflected by the planet as well as search for night airglow and planetary aurora. A magnetometer mounted on a long boom projecting from the spacecraft’s frame would characterize the interplanetary magnetic field and that of Venus. A plasma detector would study the then newly-discovered solar wind and a radiation detector to characterize the interplanetary environment and search for radiation belts near Venus analogous to Earth’s Van Allen belt. Rounding out the instrument suite was a micrometeoroid detector. This mission would not carry a camera but one was being considered for a possible follow up mission. However, if the payload margin permitted it, there was consideration given to including a polarimeter to measure the degree of polarization of the sunlight reflected from Venus through a range of viewing and lighting angles to help characterize the planet’s atmosphere and clouds.
The Mission to Venus
Launch of the Mariner A would take place from one of two pads at the new Launch Complex 36 at Cape Canaveral. About 38 minutes after liftoff, following the end of the powered ascent, the spacecraft would separate from the spent Centaur stage and deploy its high gain antenna and the pair of power-producing solar panels. At this same time, a protective cover on the particle spectrometer would be jettisoned and the instrument platform carrying the radiometers and ultraviolet spectrometer would be deployed. The first midcourse correction would be performed sometime during the first 3½ days of the mission to correct for the inevitable errors in the departure trajectory from launch. In order to minimize the effects on the spacecraft’s center of gravity, the booms for the magnetometer and plasma instrument would be deployed 84 hours after launch.
A model of the Mariner A in its initial cruise configuration before its instrument booms and platforms were deployed. Click on image to enlarge. (NASA/JPL)
During its 91 to 158-day cruise to Venus, Mariner A would return data about the interplanetary environment. A second course correction, which was deemed unlikely to be needed, would take place during the cruise. The close encounter phase of the mission, which would last 8 to 10 hours, would begin when Mariner was about 153,000 kilometers from Venus. The first task of the encounter phase would be to jettison the protective covers from the ultraviolet spectrometer and the planetary sensor on the instrument platform allowing it to slew, locate and point its instruments towards Venus. Because of the primitive state of interplanetary navigation at the time and the lack of any sort of terminal navigation capability, the closest approach would be over the day side of Venus at a comparatively distant range of 26,900 kilometers.
The 26-meter tracking antenna at the Goldstone Station that was used on early lunar and planetary missions as it appeared shortly after its completion in 1958. (NASA/JPL)
The encounter with Venus would be timed so that Mariner A could be tracked in real time using the 26-meter antenna at NASA’s Goldstone facility in California with stations in Womera, Australia and Johannesburg, South Africa also providing tracking support. Mariner A would transmit real-time telemetry during the encounter with the bulk of the science data stored on a recorder with a one million bit capacity. The recorded data would be transmitted back to Earth over the two days following the close encounter with Venus. Mariner A would then continue studying the interplanetary environment from its new solar orbit.
The Cancellation of Mariner A
Even as JPL was developing its plans to launch Mariner A to Venus, the availability of the high-performance Atlas-Centaur required for the 1962 launch opportunity was becoming increasingly doubtful. Problems in the development of the Centaur stage and its RL-10 engines coupled with issues with MSFC’s management of the program resulted in continued delays to the initial Atlas-Centaur test flight originally scheduled for December 1961 in planning documents from the beginning of that year (in the end, the Atlas-Centaur’s first test flight – a failure – did not come until May 8, 1962). Anticipating continued problems, planners at JPL began studying alternatives to the Mariner A proposal in order to get something launched to Venus in 1962 and avoid being upstaged by the Soviet Union with their own Venus launch (see “You Can’t Fail Unless You Try: The Soviet Venus & Mars Missions of 1962“).
Comparison of JPL’s Block I Ranger spacecraft (left) and the Mariner R derived from it (right). (NASA/JPL)
With no realistic options available for switching launch vehicles, attention turned towards building a smaller spacecraft sized for the more readily available (but less capable) Atlas-Agena B rocket. The focus of the new plan was to use a stripped-down version of JPL’s Block I Ranger spacecraft (see “The Prototype That Conquered the Solar System”). Development of this 307-kilogram, three-axis stabilized spacecraft was already well underway in order to serve as an engineering prototype for the Block II Rangers which were to attempt to hard land instrumented packages on the lunar surface during 1962 (see “NASA’s First Moon Lander”). Stripped of its large instrument suite and modified to increase its reliability using Mariner A technology, the Mariner R proposal (“R” standing for “Ranger”) would have a launch mass of just 203 kilograms and carry a more modest 11-kilogram instrument suite – well within the payload capability of the Atlas-Agena B.
Diagram of the Mariner R spacecraft and its Atlas-Agena B launch vehicle. Click on image to enlarge. (NASA/JPL)
When it became apparent by the second week of August 1961 that the Atlas-Centaur would not be ready in time to launch Mariner A, JPL pushed forward on Mariner R as an alternative. JPL formally proposed Mariner R on August 28 and two days later, the NASA Office of Space Flight Development announced its decision: Mariner A was cancelled. While development work on Mariner B would continue for a launch to Venus or Mars in 1964, JPL was directed to prepare a pair of the proposed Mariner R spacecraft to be launched on separate Atlas-Agena B rockets during a window extending from July 22 to August 27, 1962 – less than 11 months away. The stage was now set for what would turn out to be the first successful flyby of Venus by Mariner 2 on December 14, 1962.
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Related Reading
“The Prototype That Conquered the Solar System”, Drew Ex Machina, September 15, 2015 [Post]
“You Can’t Fail Unless You Try: The Soviet Venus & Mars Missions of 1962”, Drew Ex Machina, November 1, 2017 [Post]
General References
Edward Clinton Ezell and Linda Neuman Ezell, On Mars: Exploration of the Red Planet 1958-1978, NASA SP-4212, 1984
Irving Stone, “Mariner to Scan Venus’ Surface on Flyby”, Aviation Week, Vol. 74, No. 24, pp. 52-53, 57, June 12, 1961
Paolo Ulivi with David M. Harland, Robotic Exploration of the Solar System Part 1: The Golden Age 1957-1982, Springer-Praxis, 2007